xPower is a weighted average of your power output and serves as an alternative stress metric. It accounts for non-instantaneous physiological responses of the body to changes in intensity.
xPower is always greater than or equal to your average power. For a steady ride (or run), both will be equal. For a non-steady ride (or run) xPower is the best guess at your average power if you did a steady ride (or run) with the exact same effort.
xPace and xGAP are determined in the same way as xPower.